In recent years there have been an increased focus on capturing carbon dioxide emissions from power production based on combustion of carbon containing fuels. The emission of carbon dioxide is being considered to play an important role as a greenhouse gas and thereby influence global warming. Three main concepts have been developed to capture carbon dioxide called post-combustion, pre-combustion and oxy-fuel. Post-combustion is based on separation of carbon dioxide from exhaust from a traditional combustion process. Pre-combustion is based on transforming hydrocarbon fuel to a non-carbon containing fuel, such as hydrogen, which can be combusted without formation of carbon dioxide. The main idea of the oxy-fuel concept is to perform the combustion of a carbon containing fuel using pure oxygen and thereby obtaining an exhaust mainly comprising carbon dioxide and water. The separation of carbon dioxide and water is not very power demanding compared to the other separations. The present invention aims at provide an improved oxy-fuel based process and power plant. The water present in the exhaust gas from an oxy-fuel plant can be removed trough condensation creating a pure carbon dioxide stream applicable for storing or reuse in other processes including injection in geological formations for enhanced natural gas or oil recovery.
The oxy-fuel concept depends on the formation of pure oxygen which at present is an energy demanding process that limits the total energy efficiency of an oxy-fuel power plant. The aim of the present invention is to provide a process and a power plant with increased energy efficiency due to more energy efficient oxygen production.
Oxygen production based on membrane separation has during the later years received attention as an applicable and economically promising solution for the oxy-fuel process. Recently, there has been a rapid development of Ion Transport Membranes (ITM's) both for pure oxygen production and for applications with an oxygen consuming reaction on the permeate side.
If pure oxygen is to be produced in an ITM, there must always be a higher partial pressure of oxygen on the feed side than on the permeate side to maintain a positive flux. The difference in partial pressure constitutes the driving force of the separation. This means that if a main portion of the oxygen in the feed air is to be used in such a configuration, the air feed stream must be highly compressed, or a vacuum must be created on the permeate side.
In an ITM process, air is fed to the feed side of the membrane. The oxygen partial pressure in pure air is approximately 21% of the total pressure in the air. The partial pressure of oxygen on the permeate side must be lower than on the feed side to create a positive flux, and a high pressure difference is required for a high flux. This can be achieved by either a vacuum pump on the permeate side, a sweep stream on the permeate side, by compressing the feed air or by having an oxygen consuming reaction on the permeate side. Lowering of the energy demand for oxygen production results in an increase in the total energy efficiency of an oxy-fuel based power plant. During the ITM process the driving force, that is to say the difference in partial pressure over the membrane, is decreased as the partial pressure on the feed side is reduced when oxygen is transferred to the permeate side.
For ITM's to have an applicable efficiency, the temperature of the membrane and the incoming oxygen source should be increased, which is a power demanding process.
WO00/33942 discloses a method for recovering CO2 comprising oxygen separation from a series of mixed conducting membranes. To increase the driving force over the membranes, a sweep gas is used on the permeate side to remove the oxygen. The sweep gas loaded with oxygen is in a separate step fed to a combustion chamber where the oxygen is reacted with a carbon containing fuel. The exhaust gas from a first combustion chamber is used as sweep gas in the next membrane separation unit.
US2004/0128975 describes a low pollution power generation system involving air separation. In one embodiment air separation is obtained through a combination of membranes. Here air is separated in a first membrane unit creating an oxygen rich stream and a nitrogen rich stream. The nitrogen rich stream is introduced to the feed side of a second membrane unit creating a super rich nitrogen stream and an oxygen comprising rest stream. The oxygen comprising rest stream is returned and mixed with the incoming air stream. The rich oxygen stream is passed from the first membrane unit to the feed side of a third membrane unit creating a super rich oxygen stream and a nitrogen containing rest stream which is returned and mixed with the incoming air stream.
WO2008/074181 discloses selective oxygen-permeable membranes. In one embodiment the oxygen membrane is part of a reactor comprising two zones separated by the membrane. Oxygen containing gas is fed to the first zone and a reactant, such as a hydrocarbon, is fed to the second zone. Oxygen passes the membrane and reacts with the reactant. The reaction of oxygen with the reactant lowers the oxygen partial pressure in the second zone.
When having an oxygen consuming reaction on the permeate side then the feed side becomes oxygen depleted and the oxygen partial pressure on the feed side is reduced. In an oxy-fuel power process, it is advantageous to have higher temperature after the combustion to increase the overall efficiency of the process (according to Carnot's principle). The temperature should be as high as the turbine permits. However ITM's have temperature limitations of around 1000° C., so that if the combustion process where to be performed on the permeate side of an ITM, the low combustion temperature would result in a decrease in the total energy efficiency of the system.